The nociception receptor (NOP, previously known as the opioid receptor-like receptor ORL1) is the new, fourth member of the opioid receptor family. Although the functions of NOP and its endogenous ligand nociceptin or orphanin FQ are not well understood, this receptor system clearly plays a modulatory role in opioid-related antinociception, tolerance, and reward. These functions, in addition to the receptor's role in anxiety, learning and memory, feeding, and so on, suggest that novel small-molecule NOP ligands have potential therapeutic applications in pain, drug addiction, and other indications. In our current funding period, we discovered several novel NOP ligands and established structure-activity relationships (SAR) for binding, selectivity, and intrinsic activity for small-molecule ligands. Our SAR studies have identified molecular modifications that can change the activity profile of NOP agonists to antagonists, yielding rational drug design approaches to NOP ligands of desired activity profile. We also have the first published results of mixed NOP/opioid receptor ligands that show promise as analgesics with reduced side effects. In this competing renewal, we will build on extensive medicinal chemistry and SAR knowledge and propose receptor mutagenesis studies guided by our ligand-derived SAR to study the structural requirements for NOP receptor selectivity and activation by small molecule NOP ligands. Aim 1 of this integrated, multidisciplinary research plan will use ligand-based approaches of rational drug design and pharmacophore development to drive the design and synthesis of novel selective NOP agonists and antagonists. Novel compounds will be evaluated by receptor binding and cell-based functional assays. Aim 2 will complement these studies by using receptor-based approaches to site-directed mutagenesis of the NOP receptor to identify amino acids involved in small-molecule binding and activation. This information will also be used to aid in the design of selective agonists and antagonists. Aim 3 will investigate the in vivo actions of selective NOP agonists and antagonists. Brain penetration will be determined, as will the ability of agonists to attenuate opiate and cocaine reward using the place conditioning paradigm and the ability of antagonists to act as analgesics or potentiate opioid analgesia in acute, inflammatory, and chronic pain models. The NOP receptor has been shown to modulate opiate actions in pain and in reward associated with drugs of abuse. The work proposed in this application will discover novel compounds that can be powerful analgesics with reduced tolerance and side effects, or potential treatments for drug abuse.